Witness from trial, breaking schedule, turned up (8)
I believe the answer is:
attestor
'witness' is the definition.
(I know that witness can be written as attestor)
'trial breaking schedule turned up' is the wordplay.
'trial' becomes 'test' (synonyms).
'breaking' indicates putting letters inside.
'schedule' becomes 'rota' (a rota is a schedule).
'turned up' is a reversal indicator.
'rota' reversed gives 'ator'.
'test' put within 'ator' is 'ATTESTOR'.
'from' acts as a link.
(Other definitions for attestor that I've seen before include "Demonstrator" , "One giving evidence" , "he'll swear!" , "> witness" , "One who enrolled" .)